1. Field of the Invention
The present invention relates to a power circuit of an air conditioner for an electric vehicle, and, more specifically, to a power circuit supplying a high voltage direct current from a high voltage power source to a drive circuit for driving a compressor of an air conditioner for the electric vehicle.
2. Description of the Related Art
A conventional power circuit of an air conditioner for an electric vehicle is constructed, for example, as depicted in FIG. 3. In FIG. 3, air conditioner 21 includes a compressor 22, a compressor motor 23 connected to compressor 22, a power circuit 24 supplying a high voltage direct current from a high voltage power source 25 to a drive circuit comprised of an inverter 26, a positive side current interrupting device composed of a switch 27a interposed in a positive side coupling 28a from high voltage power source 25, a negative side current interrupting device composed of a switch 27b interposed in a negative side coupling 28b, and a short detecting circuit 29 connected between the negative side coupling 28b and ground 30 for determining the existence of a short by detecting a short resistance therebetween. High voltage power source 25 also supplies a high voltage direct current to an operation control device 31 that controls the operation of the electric vehicle.
Power circuit 24 has a diode D coupled in the positive side coupling 28a of the positive side of high voltage power source 25, a capacitor 32 connected between the positive side coupling 28a and the negative side coupling 28b for storing current, a discharge resistor 33 connected between positive side coupling 28a and negative side coupling 28b for discharging capacitor 32, and a charge switch 34 connected in series with a charge resistor 35 and between both ends of switch 27a. Switch 27a and charge switch 34, connected to charge resistor 35, are connected in parallel relative to diode D. Diode D prevents a reverse connection, i.e., interrupting the circuit on the air conditioner side 61 when the positive side and the negative side of power source 25 are reversely connected. Power circuit 24 further includes a motor control circuit 36. Motor control circuit 36 controls the opening and closing operation of switches 27a and 27b, charge switch 34, and the driving of inverter 26, in response to the signals from short detecting circuit 29 or the detected charged condition of capacitor 32.
Between motor control circuit 36 and each portion of the circuit are connected: signal wires S2 and S3 connected to short detecting circuit 29 for sending a drive signal and receiving a detected short signal, a signal wire S4 connected to capacitor 16 for inputting the detected charged voltage, a signal wire S5 connected to inverter 26 for transmitting a dive control signal, a signal wire S6 connected to charge switch 34 for transmitting an open/close operation signal, a signal wire S7 connected to the positive side switch 27a for transmitting an open/close operation signal, and a signal wire S8 connected to the negative side switch 27b for transmitting an open/close operation signal.
In power circuit 24, the charging of capacitor 32 occurs as follows. When the high voltage direct current from high voltage power source 25 is initially transmitted into air conditioner 21, if the high voltage direct current is directly transmitted to diode D by closing switches 27a and 27b, which are controlled by motor control circuit 36, excessive current may instantaneously flow in diode D, thereby breaking diode D and causing the life of capacitor 32 to potentially decrease. Therefore, motor control circuit 36 closes charge switch 34 and negative side switch 27b by the operation signals transmitted through wires S6 and S8 and opens positive side switch 27a in response to the operation signal transmitted through wire S7. The current from high voltage power source 25 flows through charge resistor 35 and thus capacitor 32 is gradually charged. After capacitor 32 is sufficiently charged, positive side switch 27a is closed in response to the operation signal transmitted through wire S7. In motor control circuit 36, when the capacitor 32 is completely charged, as recognized through signal wire S4, charge switch 34 is then opened in response to the operation signal transmitted through wire S6. After positive side switch 27a is closed in response to the operation signal transmitted through wire S7, inverter 26 is driven in response to the operation signal transmitted through wire S5.
When a short in the air conditioner side 61 is detected by short detecting circuit 29, motor control circuit 36 opens all switches 34, 27a and 27b in response to the operation signals transmitted through wires S6, S7 and S8. Motor control circuit 36 then drives short detecting circuit 29 in response to the driving signal transmitted through wire S2, and receives a short detection signal from short detecting circuit 29 through wire S3.
In such a conventional power circuit 24 for electric vehicles, because diode D must have a sufficiently large capacity to withstand a high voltage direct current from high voltage power source 25, a large diode D of significant cost is required. Moreover, because charge switch 34 and the positive side and negative side switches 27a and 27b are required, and signal wires S6, S7 and S8 connect between the respective switches and motor control circuit 36 to prevent reduction of the life of capacitor 32. Therefore, the number of parts required is also high and the circuit is more complicated, thereby further increasing the cost and size of the circuit.
Moreover, although a high voltage power source mounted in an electric vehicle is usually insulated, because an electric shock may occur if the insulation is broken, short detecting circuit 29 is needed in the circuit. After recognition of no detected short, inverter 26 may be driven in response to the control signal of motor control circuit 36 in order to prevent occurrence of an electric shock. In such a system, when short detection is carried out on only the air conditioner side 61, the circuit of the air conditioner side 61 may be interrupted from the circuit of the operation control device side 51. Therefore, at least negative side switch 27b is required to achieve such an interruption. Thus, these switches are necessary in the known circuit.